1. Field of the Invention
The present invention relates to a high frequency electric wire, and more particularly to an electric wire which can be used as a wiring material, a winding material or the like for high frequency.
2. Description of the Prior Art
(Technological Background)
For example, a high frequency alternating current method is adopted in a non-contact power feeding device, but the increase in frequency is remarkable in the frequencies used.
In other words, in the non-contact power feeding device which supplies power through an air gap to a power receiving side coil mounted on an electric vehicle and the like from a fixed power feeding side coil based on the mutual induction action of the electromagnetic induction, the high frequency alternating current method is adopted from the viewpoint of the needs such as improvement of battery charging efficiency, a larger gap size, and a light and small coil. In view of these needs, the high frequency of about 20 kHz is now used, but it is projected to use a high frequency between 50 kHz and 100 kHz or above 100 kHz in the future.
For example, even in audio equipment, game equipment, personal computers and various types of other electrical appliances other than the non-contact power feeding device, the needs for adoption of the high frequency alternating current method and the increase in useful frequency are remarkable.
(Description of the Prior Art)
In such a non-contact power feeding device and other equipment, an electric wire 1 or 2 as shown in FIG. 4 has been typically used as a wiring material, a winding material or the like, for example, as a circuit cable or a coil.
The electric wire 1 as shown in FIG. 4A is an ordinary electric wire of this kind. The electric wire 1 is made by bundling and twisting a large number of wires 3 made of a solid wire, and insulating the entire outside surface by an outer sheath S. The electric wire 2 as shown in FIG. 4C is referred to as a litz wire. The electric wire 2 is made, as shown in FIG. 4D, by bundling and twisting a large number of wires 5 of which the outer periphery is coated with an insulating film 4 such as enamel, and then insulating the entire outside surface by an outer sheath S. The individual wire 5 is made of fine wire of which the diameter is 200μ or less.
FIGS. 4A and 4C are schematic views, wherein one electric wire 1 or 2 consists of seven wires 3 or 5. It is however to be noted that, for practical purposes, one electric wire 1 or 2 consists of several tens of wires to several tens of thousands of wires 3 or 5.
(Conventional Technical Literature Information)
(Patent Document)
Such a non-contact power feeding device is disclosed in patent document 1. The electric wire 2, that is, the litz wire is disclosed, for example, in patent document 2.    Patent Document 1: Japanese Unexamined Patent Publication No. 2008-087733    Patent Document 2: Japanese Unexamined Patent Publication No. Hei 5-263377
3. Problems to be Solved by the Invention
(Problems)
It has been pointed out that the conventional technology has the following problems.
(First Problem)
In the case where the electric wire 1 (ordinary electric wire) as shown in FIG. 4A is used as a circuit cable for a high frequency alternating current or for the high frequency power transmission, there are problems in high frequency alternating current resistance due to a skin effect, Joule heat loss, and power loss.
In other words, when the electric wire 1 (ordinary wire) is used, the higher the frequency, the higher the influence of the skin effect and as a result, the frequency characteristics deteriorate, the high frequency alternating current resistance increases, the square of the Joule heat loss increases and as a result, the power loss becomes obvious.
The skin effect will now be described. As is well known, the higher the frequency, the easier the alternating current flows on a surface side of the electric wire 1 by a mutual action between a generating alternating magnetic flux and an electric current and the electric current density is concentrated on the surface side. As a result, the high frequency alternating current flows intensively on part of the total cross-sectional area of the electric wire 1, that is, on an extremely-thin section of the surface side and the alternating current resistance value increases due to reduction of the current-carrying area.
(Second Problem)
As is well known, the electric wire 2 (litz wire) as shown in FIG. 4C has been developed for high frequency to solve the problems of the electric wire 1 (ordinary electric wire) described above.
Namely, the electric wire 2 is made by bundling, for example, thousands of extra-fine wires 5, of which the diameter is 200μ or less, coated with an insulating film 4. With this structure, the electric wire 2 is provided in such a manner that a surface area of each wire 5, that is, a surface area of its outer peripheral surface, can be increased in total. In other words, the electric wire 2 is provided to disperse, split and reduce the concentration of the electric current density due to the skin effect by increasing the number of the surfaces, that is, the outer peripheral surfaces of the individual wire 5 as compared to the electric wire 1 described above, thereby increasing the surface area accordingly.
However, even in the electric wire 2, it has been further required to suppress and decrease the high frequency alternating current resistance, to lower the Joule heat loss, and to reduce the power loss in accordance with a higher level of the useful frequency. In this manner, it has been desired to improve the frequency characteristics of the electric wire 2.
It has also been pointed out that the electric wire 2 has a problem from a cost viewpoint. First, the high frequency alternating current flows only on the surface of each wire 5 based on the skin effect and it does not flow in the center thereof. In this manner, it has been pointed out that the central side of the total cross-sectional area of each wire 5 except for the extremely-thin surface side is a non-use area, that is, a wasteful area on which the electric current does not flow and exhibits greater loss capability in material costs (as in the electric wire 1 described above). It has also been pointed out that the production costs become higher because the outer periphery of each wire 5 must be coated with an insulating film 4.